Experiments are directed toward understanding processes by which regulation of the immune system is controlled at the cellular level. The integrated roles of regulatory cells such as thymus-derived helper and suppressor T-cells, bone-marrow derived antibody-forming B-cells and accessory macrophages are being delineated. Specifically, we are seeking an understanding of the relationship of suppressor and helper cells and their modes of regulating antibody-forming B-cells. We have demonstrated that a streptococcal exotoxin (SPE) inhibits preferentially an activity which has been ascribed to suppressor cells. SPE is therefore potentially a useful probe for the study of suppressor cells. Subpopulations of immunocytes fractionated from T-cell sufficient plus/nu donor mice are treated appropriately with SPE or other mitogens in vitro. The cells are then tested for their capacity to complement the antibody responses of T-cell deficient nude (nu/nu) mice or their spleen lymphocytes (B-cells) in vitro. This study involving SPE may also derive relevant information about the sequelae of acute streptococcal disease, as well as fundamental information on the cellular biology of the immune system.